Generally, determination of at least one characteristic of an environment proximate to a vibratory source may be important for any number of reasons. For example, it is often of interest to ascertain at least one characteristic of an environment proximate to pipes, conduits, ducts, or other structural elements that extend within or proximate to a region of interest such as, a petroleum well, a hazardous waste site, a coal bed, or any other region of interest.
For instance, identification of subsurface rock properties may be valuable in oil well production since properties such as porosity and permeability may influence oil recovery. Conventional methods may employ correlations based upon acoustic travel times and bulk density to measure rock properties such as bulk compressibility, shear modulus, Young's modulus, and Poisson's ratio. Additional conventional methods may employ magnetic resonance or resistivity measurements for determining rock properties. Alternatively, other conventional methods directly measure rock properties through testing of samples that are taken from the well location and transported to a laboratory. However, such testing may be relatively difficult and time consuming, may only approximate in situ conditions, and may also be expensive.
Alternative conventional methods may employ an orbital vibrator for providing a seismic signal or wave from which a property of a subterranean formation may be determined. The use of orbital vibrators as seismic waveform generators is well known in the area of geophysical prospecting for oil, or the like. Typically, an orbital vibrator may take the form of an eccentric rotating mass enclosed within a shell or tube which may be used in various combinations to generate the desired seismic signal(s). Examples of orbital vibrators are disclosed by U.S. Pat. No. 4,709,362 to Cole, U.S. Pat. No. 4,874,061 to Cole, U.S. Pat. No. 5,229,552 to Cole, and U.S. Pat. No. 5,321,213 to Cole.
In addition, U.S. Pat. No. 4,419,748 to Siegfried, discloses a continuous wave sonic logging method in which a continuous sine wave at a single frequency is emitted and received, and a spatial Fourier transform is performed over the receiver array. The resulting spatial frequency components are then used to indicate the velocities of various sonic paths. This would require numerous logging runs for the dispersive waves, for which the wave characteristics are functions of frequency. Since the logging time is a costly factor in wire line logging services, the method is not practical for logging dispersive waves.
U.S. Pat. No. 4,874,061 to Cole, mentioned above, relates to an apparatus for simultaneously generating elliptically polarized seismic shear waves and compression waves downhole for performing reverse vertical seismic profiles. The apparatus includes an elongate frame for support in the borehole and the frame includes a drive means energizable to impart an orbital motion to at least a portion of the frame to generate an orbital shear wave.
U.S. Pat. No. 2,244,484 to Beers relates to a method for seismically determining physical characteristics of subsurface formations which includes generating a sound in the vicinity of a formation. More specifically, a method of seismically determining physical characteristics of geologic strata is disclosed which includes propagating sound waves in the immediate vicinity of the formation or stratum, measuring the velocity of propagation of the sound through the formation and indicating the velocity at the surface. The characteristics of the formation may be readily determined by measuring the time required for the waves to travel through the formation.
U.S. Pat. No. 4,802,144, to Holzhausen et al. discloses a method employing a principle that the growth of a hydraulic fracture increases the period of free oscillations in the well connected to the fracture. Holzhausen discloses that hydraulic fracture impedance can be defined in terms of the hydraulic resistance and the hydraulic capacitance of a fracture. Further, fracture impedance can be determined directly by measuring the ratio of down hole pressure and flow oscillation or indirectly from well head impedance measurements using impedance transfer functions. Because impedance is a function of fracture dimensions and the elasticity of the surrounding rock, impedance analysis of a seismic wave emanating from the borehole can be used to evaluate the geometry of the fracture by analyzing the data which results from free and forced oscillations in the well, and looking for a match between the data and theoretical models of projected shapes of the fracture.
U.S. Pat. No. 5,121,363 to Benzing discloses a method for identifying fractured rock formations and determining their orientation by employing two orthogonal motion sensors which are used in conjunction with a downhole orbital vibrator. The downhole vibrator includes a device for orienting the sensors. The output of the sensors is displayed as a lissajou figure, wherein the shape of the figure may change responsive to encountering a subsurface fracture by the apparatus. Thus, the apparatus and method may be used to identify fractured rock formations and may enable determination of the azimuthal orientation of the fractures.
In view of the foregoing, it would be advantageous to provide improved methods and apparatuses which facilitate determination of at least one property of an environment proximate to a vibratory source.